It has long been known to separate air in multiple column arrangements having higher and lower pressure columns to produce nitrogen and oxygen-rich fractions and an argon column to rectify an argon and oxygen-containing vapor stream taken from the lower pressure column to produce an argon-rich fraction.
In such air separation systems, air is compressed and purified to remove higher boiling impurities such as carbon monoxide, carbon dioxide and water. The resultant compressed and purified stream is cooled in a main heat exchanger to a temperature at or near the dew point of air and the resultant cooled stream is then introduced into the higher pressure column. The air is rectified in the higher pressure column to produce a nitrogen column overhead and a crude liquid oxygen column bottoms. The crude liquid oxygen column bottoms is then further refined within the lower pressure column to produce a liquid oxygen column bottoms and a nitrogen-rich column overhead.
The higher and lower pressure columns are operatively associated with one another in a heat transfer relationship by means of a condenser-reboiler that vaporizes a liquid oxygen column bottoms produced in the lower pressure column against condensing nitrogen column overhead in the higher pressure column to reflux the higher pressure column. A stream of the condensed nitrogen column overhead is then introduced into the lower pressure column for reflux purposes.
A vapor stream containing oxygen and argon is removed from the lower pressure column and is then rectified in the argon column to produce an argon-rich column overhead that can be extracted as a product or further refined to produce the argon product. The argon column is refluxed by a condenser. A stream of the crude liquid oxygen column bottoms is expanded to the pressure of the lower pressure column, thereby to also lower its temperature. Thereafter, at least a portion of this stream is then introduced into the condenser to condense some of the argon-rich column overhead. The resultant vaporization within the argon condenser produces vapor and liquid phases that are subsequently introduced into the lower pressure column.
The introduction of the vapor fraction derived from the crude liquid oxygen being introduced into the lower pressure column increases the nitrogen traffic within the lower pressure column and therefore decreases the amount of argon being washed down the column to the point at which the argon and oxygen-containing vapor stream is taken for further refinement in the argon column. This problem is exacerbated when liquid oxygen and nitrogen products are to be produced at pressure. For example, when liquid oxygen is taken for production of an oxygen product at pressure, a liquid oxygen stream may be pumped and then vaporized in the main heat exchanger. For such purposes part of the air is compressed in a booster compressor to thermally compensate for such vaporization. Liquefaction of the air taken for such purposes results in less nitrogen vapor being produced in the higher pressure column and therefore, less reflux to the lower pressure column.
In order to combat this problem, U.S. Pat. No. 5,386,691 provides for a portion of the vapor fraction produced in the argon column condenser to be valve expanded and redirected to the waste nitrogen stream. In so doing, the reflux ratio in the upper section of the lower pressure column is increased thereby increasing argon recovery because there is less vapor traffic in the lower pressure column due to a reduction in the introduction of nitrogen-rich vapor into the lower pressure column. This improves the liquid to vapor ratio in the lower pressure column above the point at which the argon and oxygen containing stream is taken for rectification in the argon column.
As will be apparent from the discussion below, the present invention provides an improved method of separating air in a multiple column arrangement in which argon recovery is improved by increasing the liquid to vapor ratio within the uppermost portion of lower pressure column.